Wafer carrier mapping is an important element in semiconductor manufacturing. Each wafer may include many semiconductor chips and represent thousands of dollars worth of materials and time. Wafer carrier mapping is a process wherein errors can be detected and addressed before significant manufacturing losses occur. As used herein, “mapping wafers” means scanning a set of wafers in either a transport or storage cassette or pod, and determining which slots in the cassette or pod have wafers in them and whether any of the wafers are incorrectly placed. Preferably, a device in accordance with embodiments of the present invention is mounted on a wafer-handling robot (a known robotic system used in wafer manufacture), and maps the wafers as the robot arm moves in a generally vertical path in front of the cassette or pod.
Wafer carrier mapping systems use high-quality images to calculate location, thickness and correct slotting of one or more wafers in a carrier. One potential source of error present during the acquisition of such high-quality images is due to undesirable illumination being sensed by the imaging array. The only rays of illumination that are desirable during image acquisition are those that are reflected from a wafer surface. However, sometimes, a ray can be reflected from a non-wafer surface, such as a surface within a wafer cassette and generate erroneous image information. Thus, there is a need to provide a wafer carrier mapping sensor assembly that is less sensitive to the effects of undesirable illumination.